The invention relates to a process for the preparation of polymers of monoalkenyl arenes and/or conjugated dienes.
Block copolymers have been developed rapidly within the recent past, the starting monomers usually being monoalkenyl arenes such as styrene or alphamethylstyrene block polymerized with conjugated dienes such as 1,3-butadiene and isoprene. A typical block copolymer of this type is represented by the structure polystyrene-polybutadiene-polystyrene (SBS). When the monoalkenyl arene blocks comprise less than about 55% by weight of the block copolymer, the product is essentially elastomeric. Moreover, due to their peculiar set of physical properties they can be referred to more properly as thermoplastic elastomers. By this is meant polymers which in the melt state are processable in ordinary thermoplastic processing equipment but in the solid state behave like chemically vulcanized rubber without chemical vulcanization having been effected. Polymers of this type are highly useful in that the vulcanization step is eliminated and, contrary to scrap from vulcanized rubbers, the scrap from the processing of thermoplastic elastomers can be recycled for further use. Such block copolymers may also be hydrogenated to produce polymers having improved oxidative stability, along with other improved properties.
Triblock or multiblock copolymers such as S-B-S or styrene-ethylene/butylene-styrene (S-EB-S) have a balance of high tensile strength and low modulus that makes them excellent materials for applications such as adhesives, footwear and blending with other thermoplastics and thermosets. Properties such as creep, hysteresis and tensile strength improve as molecular weight increases for triblock copolymers. However, production is also increasingly difficult as molecular weight increases because solution viscosity increases markedly with molecular weight and time required for polymerization increases. As the time in the reactor increases, impurities and thermal termination cause more chains to be prematurely terminated. Therefore, it would be advantageous to make triblock copolymer and multiblock copolymers by coupling diblock molecules of one half the final molecular weight (or less in the case of stars) after the polymerization has been finished in the plant. Furthermore, most of these polymers are sold as a component for blending.
Therefore, a convenient place for coupling would be in the extruder during blending.
A novel process has now been discovered that permits the production of such coupled block copolymers in a more efficient manner.
The present invention provides a process for the preparation of a polymer which may be coupled at temperatures above 200 °C, which process comprises:-
- a) solution polymerizing a conjugated diene and/or a monoalkenyl arene under polymerization conditions at a temperature between -75 °C and +150 °C with an organomonolithium compound, therein forming living polymeric arms; and
- b) contacting the solution containing said living polymeric arms with an effective amount of an arylcyclobutene monomer of the general formula
Preferably, the resulting polymer is contacted with hydrogen and a hydrogenation catalyst under hydrogenation conditions and at a temperature between -75 °C and +150 °C, therein reducing by hydrogenation at least 80% of the aliphatic unsaturation of the polymeric arms while reducing less than 20% of the aromatic unsaturation and forming hydrogenated polymeric arms.
There is a number of possible advantages to the present invention. With the present invention it may be possible to produce higher molecular weight polymers having improved creep resistance, elasticity, strength, low permanent set and improved blending capability with other polymers.
It may also be possible with the present invention to debottleneck production plants and improve efficiency. For example, using conventional coupling agents, a limiting factor is the solution viscosity after coupling. However, with the present invention "coupling" does not occur until the polymer is heated above 200 °C. Accordingly, higher solids levels can be maintained in the polymerization (coupling) reactors, and actual coupling can be delayed until just before polymer recovery (e.g. in an extruder) or even later times.
Further, with diblocks terminated with arylcyclobutene, one might be able to have diblock rheology in the first zone of an extruder and triblock rheology at the exit of the extruder. Such a combination might prove very beneficial for blending and interpenetrating network formation.
The preferred group of acyclic conjugated dienes that can be polymerized into the polymer chain P are those containing 4-8 carbon atoms per molecule. Examples for such dienes are 1,3-butadiene, 2,3-dimethyl-1,3-butadiene, piperylene, 3-butyl-1,3-octadiene, isoprene and 2-phenyl-1,3-butadiene.
Monoalkenyl arenes that can be polymerized together with the dienes to form the polymer chain P are preferably styrene, the methylstyrenes, particularly 4-methylstyrene, the propylstyrenes, particularly 4-propylstyrene, vinylnaphthalene, particularly 1-vinylnaphthalene, cyclohexylstyrenes, particularly 4-cyclohexylstyrene, p-tolylstyrene, and 1-vinyl-5-hexylnaphthalene.
The polymer chains P can be homopolymers of the diene monomers defined or can be copolymers of diene monomers and monoalkenyl-substituted aromatic monomers. These copolymers, in turn, can be random or tapered copolymers, as well as block copolymers of these various monomers. The presently preferred monomers are isoprene, 1,3-butadiene and styrene. The present preferred polymer chains P are those in which the conjugated dienes are present in a major amount and the monovinyl-substituted arenes are present in a minor amount.
The presently preferred polymer is one that is obtained by endcapping according to the present invention a living lithium metal-terminated polymer being a homopolymer of alkadienes having 4 to 12 carbon atoms per molecule or a copolymer of at least one diene of 4 to 12 carbon atoms per molecule.
The molecular weight of the polymers prepared according to the present invention can vary in broad ranges. For the usual applications of the coupled polymers, the number average molecular weight will be in the range of 6,000 to 2,000,000.
Those polymers in which the polymer chain P has a structure A―B― so that B is attached to the endcapping agent of the present invention, and in which A represents a block of monoalkenylarenes, preferably a polystyrene block, and B represents a block that confers rubbery properties to the polymer chain, such as a polydiene block, a copolymer block of a diene and a monoalkenyl-substituted arene, or a combination of such blocks constitutes a presently preferred polymer. Such a polymer exhibits properties both of an elastomer and of a thermoplastic polymer. Therefore, such polymers can be formed into articles by standard procedures known for producing articles from thermoplastic polymers while the finished article exhibits elastomeric properties.
Furthermore, specific polymers constituting preferred embodiments of this invention are those obtained by reactions and procedures disclosed in detail in the following description of a process to make these polymers.
In accordance with a further embodiment of this invention, there is provided a process for producing the polymer as defined above. This process includes basically at least two steps. The first step is the step in which a living polymer having the formula P-Li is produced. The second step is that in which the living polymer is further reacted with the arylcyclobutene derivative as defined further below.
The first step of this process is carried out by reacting a mono-functional lithium metal initiator system with the respective monomer or monomers to form the living polymer chain P-Li. This polymerization step can be carried out in one step or in a sequence of steps. In the case where the polymer chain P is a homopolymer or a random or tapered copolymer of two or more monomers, the monomers are simultaneously polymerized with the lithium metal initiator. In the case where the polymer chain P is a block copolymer comprising two or more homopolymer or copolymer blocks, these individual blocks can be generated by incremental or sequential monomer addition.
The monomers that are generally employed, as well as the monomers that are preferably used have been defined above in connection with the polymers prepared according to the present invention. These monomers are also preferred for the process of the present invention.
The lithium metal-based initiator systems used in the first step of the process to make the coupled polymers of this invention are based on lithium having the general formula R¹Li wherein R¹ is a hydrocarbyl radical of 1 to 20 carbon atoms. Examples of such lithium initiators are methyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, t-octyllithium, n-dodecyllithium, n-eicosyllithium, phenyllithium, naphthyllithium, p-tolyllithium, 4-phenylbutyllithium, cyclohexyllithium, and 4-cyclohexylbutyllithium. The amount of the organolithium initiator employed depends upon the desired properties of the polymer, particularly the desired molecular weight. Normally the organomonolithium initiator is employed in the range of about 0.1 to 100 millimol per 100 g of total monomers.
The polymerization reaction is carried out in the presence of a hydrocarbon diluent or solvent. Preferably the hydrocarbon diluent is a paraffinic, cycloparaffinic or aromatic hydrocarbon having 4-10 carbon atoms per molecule or a mixture of such diluents. Examples for the diluent are n-hexane, n-heptane, 2,2,4-trimethylpentane, cyclohexane, benzene and toluene. The reaction is generally carried out with a weight ratio of diluent to monomers exceeding 1. Preferably the diluent is employed in a quantity between about 400 to about 1500 parts by weight per 100 parts by weight of total monomers. In one embodiment, a minor amount (0.1 to 10% by volume) of a polar compound is added to the diluent. Specific examples of polar compounds include dimethyl ether, diethyl ether, ethyl methyl ether, ethyl propyl ether, di-n-propyl ether, di-n-octyl ether, dibenzyl ether, diphenyl ether, anisole, tetramethylene oxide (tetrahydrofuran), 1,2-dimethoxyethane, dioxane, paraldehyde, dimethyl sulphide, diethyl sulphide, di-n-propyl sulphide, di-n-butyl sulphide, methyl ethyl sulphide, dimethylethylamine, tri-n-propylamine, tri-n-butylamine, trimethylamine, triethylamine, N,N-dimethylamine, pyridine, quinoline, N-ethylpiperidine, N-methyl-N-ethylaniline and N-methylmorpholine. It is to be understood also that mixture of polar compounds can be employed in the practice of the present invention. A preferred polar compound is diethyl ether. In the case where the polymer chain P is a homopolymer or a random or tapered copolymer of two or more monomers, the monomers are simultaneously polymerized with the organolithium initiator. In the case where the polymer chain P is a block copolymer comprising two or more homo-or copolymer blocks, these individual blocks can be generated by incremental or sequential monomer addition.
The polymerization reaction in step 1 usually occurs within a period of time ranging from a few minutes up to about 6 hours. Preferably, the reaction is carried out within a time period of about 10 minutes to about 2 hours. The polymerization temperature is not critical but must be below the temperature at which the arylcyclobutene is activated and is preferably in a range of about 40 °C to about 90 °C.
Preferred alkylene groups represented by X in the general formula II and alkyl groups represented by R and R¹ in the general formula II have 1 to 6 carbon atoms. Examples of X are methylene, ethylene, propylene and butylene groups and examples of R and R¹ are methyl, ethyl, propyl and butyl groups. Preferred arylene groups are phenylene groups and preferred aryl groups are phenyl groups.
Z in the formulae I and II may be any arylcyclobutene group. An arylcyclobutene refers herein to an aryl group which contains one or more cyclobutene rings fused to an aromatic ring. An aryl group refers to any aromatic moiety as defined hereinbefore. Preferred aromatic moieties include benzene, naphthalene, phenanthrene, anthracene, biaryl moieties or two or more aromatic moieties joined by an alkyl or cycloalkyl group. Examples of 2 ore more joined aromatic moieties are diphenyl alkanes, and diphenyl cycloalkanes. Benzene is the most preferred aromatic moiety.
For the arylcyclobutene group Z, the aromatic moiety ring may be non-substituted or substituted with a wide variety of substituents. For use here, however, such substituents must be stable to the reaction conditions employed and not subject to side reactions. Examples of such substituents include but are not limited to lower alkyl, e.g. methyl, ethyl, butyl; aryl, e.g., phenyl, tolyl; halo, e.g. chloro, bromo, iodo; and lower alkoxy, e.g. methoxy, ethoxy.
Preferably the arylcyclobutene is a non-substituted cyclobutene ring and a non-substituted aromatic moiety. The most preferred embodiment of arylcyclobutene group Z is benzocyclobutene.
In a much preferred embodiment the arylcyclobutene monomer has the formula
where Z represents benzocyclobutene and R represents hydrogen, i.e. 4-formylbenzocyclobutene.
The amount of benzocyclobutene monomer employed is 1 to 3 mmol per mmol of the organolithium initiator. The temperature of contacting is typically the same as polymerization, i.e., -75 °C to +150 °C.
After contacting with the arylcyclobutene monomer (resulting in polymer arms endcapped with arylcyclobutene), the polymer may be recovered by treating the reaction mixture with terminating agents containing active hydrogen atoms such as alcohols or water or aqueous acid solutions or mixtures thereof. It is usually preferred to add an antioxidant to the reaction mixture before isolation of polymer.
The polymer is separated from the reaction mixture by standard techniques, such as steam stripping or coagulation with a stable nonsolvent such as an alcohol. The coagulated or stripped polymer is then removed from the resulting medium by, e.g., centrifugation or extrusion. Residual solvent and other volatiles can be removed from the isolated polymer by heating, optionally under reduced pressure or in a forced air flow.
If desired the polymer (P-Li) may be hydrogenated prior to recovery.
Hydrogenation of the block copolymers is preferably effected by use of a catalyst comprising the reaction products of an aluminium-alkyl compound with nickel or cobalt carboxylates or alkoxides under such conditions as to substantially completely hydrogenate at least 80% of the aliphatic double bonds while hydrogenating no more than 20% of the alkenyl arene aromatic double bonds. Preferred block copolymers are those where at least 99% of the aliphatic double bonds are hydrogenated while less than 5% of the aromatic double bonds are hydrogenated.
With regard to block copolymers of the general formula (A―B)̵Y where Y is the arylcyclobutene endcapping agent, the average molecular weights of the individual blocks will vary within certain limits. In most instances, the monoalkenyl arene blocks will have number average molecular weights in the order of 5,000-125,000, preferably 7,000-60,000 while the conjugated diene blocks either before or after hydrogenation will have average molecular weights in the order of 10,000-300,000, preferably 30,000-150,000. The total average molecular weight of the block copolymer is typically in the order of 25,000 to about 350,000, preferably from about 35,000 to about 300,000. These molecular weights are most accurately determined by tritium counting methods or osmotic pressure measurements. The proportion of the monoalkenyl arene blocks should be between about 8 and 65% by weight of the block copolymer, preferably between about 10 and 40% by weight.
A key aspect of the present invention is that the polymer capped with arylcyclobutene can be heated to temperatures above 200 °C, wherein dimerization and oligomerization of arylcyclobutene is activated to give higher molecular weight coupled products.
The following scheme shows the various steps in the process according to the present invention and the final polymer as recovered.
The invention is further illustrated by means of the following Examples.
A key aspect of the present invention deals with the ring-opening of the benzocyclobutene monomers to reactive o-quinodimethanes. In this embodiment, half-life values for the parent benzocyclobutene are calculated and summarized in Table 1 hereinafter, based on activation parameters reported in W.R. Roth et al Chem. Ber. III, 3892-3903 (1978). The results suggest that reactive oligomers and polymers containing benzocyclobutenes which are not substituted at the cyclobutene ring would have long shelf-life and good reactivity at 200-250 °C.
In this Example, coupled polymers are prepared by using 4-formylbenzocyclobutene as the arylcyclobutene monomer.
To a hot solution of hexamethylene tetramine (14 g) and sodium iodide (16 g) in 140 g of ethanol was added 4-chloromethylbenzocyclobutene (15.2 g) with stirring. After standing at ambient temperature for 2-3 h, the mixture was diluted with 150 ml of water and then heated under reflux for 2 h. The resulting mixture was steam distilled and the distillate was extracted three times with ether. The combined ether extract was washed with water, dried over magnesium sulphate, and fractionally distilled to give 4-formyl-benzocyclobutene: ¹H NMR (CDCl₃) δ 3.17 (s,4), 7.15 (d,1), 7.51 (s,1), 7.67 (d,1) and 9.86 ppm (s,1).
To a solution of styrene (50 g) in cyclohexane (500 ml) was added s-butyllithium (3.3 mmol). After heating at 60 °C for 4 h, the orange polystyryllithium was titrated with a solution of 4-formylbenzocyclobutene in cyclohexane until the orange colour disappeared. The product was isolated by precipitation from 2-propanol and dried at sub-atmospheric pressure at 65 °C to give 33 g of a white powder. GPC analysis of the product showed three polystyrene peaks with the following distribution of peak molecular weight (MW) and % peak area: 10,300 (79%), 18,700 (20%), and 29,000 (1%). The formation of higher MW polystyrenes was probably due to the presence of oxygen or unidentified impurities in the endcapping solution which induced coupling of the living polystyryllithium.
A sample of benzocyclobutene-capped polystyrene synthesized as described hereinbefore was subjected to isothermal dynamic shear in a Rheometric mechanical spectrometer at 250 °C for 30 min. GPC analysis showed that the amount of coupled polystyrene (dimers, trimers, tetramers, etc.) increased from 21% to 58% after heat treatment.
Heat treatment of a hydrogen-capped polystyrene identical to the Example described hereinbefore showed no increase of coupled polystyrene.